Though he would certainly put up a great fight, Darkseid is a character who can transform and also adapt from his opponent in a way that Thanos simply cannot.
His strength is undeniable, making Thanos look like a toothpick in comparison, and you can plainly see that Darkseid is a character with great armies and no real moral compass.
Even Thanos has a bit of a heart, and that’s something that’s rarely found in Darkseid’s immortal soul.
Nevertheless, Darkseid is a character with such vast technology, incredible powers and monumental capabilities that it’s hard to believe that Thanos, for all his efforts, would be able to defeat Darkseid.